Gel electrolyte battery

ABSTRACT

A gel electrolyte battery in which adhesion between a layer of an active material of an electrode and a gel electrolyte is raised to provide for sufficient mobility of lithium ions, and a method for the preparation of the gel electrolyte battery. The gel electrolyte battery is comprised of a battery device accommodated in an exterior material of a laminated film and sealed in it on heat fusion. The method for the preparation of the gel electrolyte battery includes a battery device preparation step of layering a positive electrode and a negative electrode via a gel electrolyte to form the battery device, an accommodating step of accommodating the battery device from the battery device preparation step in the laminated film, and a heating step of heating the battery device, accommodated in the laminated film in the accommodating step, under a pressured state.

RELATED APPLICATION DATA

The present application claims priority to Japanese Application No.P2000-081860 filed Mar. 17, 2000, and is a divisional of U.S.application Ser. No. 09/768,093, filed Jan. 23, 2001 now U.S. Pat. No.6,755,873, both of which are incorporated herein by reference to theextent permitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a gel electrolyte battery having a gelelectrolyte.

2. Description of Related Art

As a power source for a portable electronic equipment, such as aportable telephone set, a video camera or a notebook personal computer,the importance attached to batteries is increasing. For reducing thesize and weight of the electronic equipment, such a battery is desiredwhich not only is of a large capacity but also is lightweight andspace-saving. Viewed in this light, a lithium battery, having a highenergy density and high output density, is suited very much. A lithiumbattery, employing a carbon material as a negative electrode material,has an average discharge voltage of not lower than 3.7 V and undergoeslesser cyclic deterioration during charging/discharging, so that it hasa merit that a high energy density can readily be realized.

The lithium battery is desired to exhibit flexibility and shape freedom,and lithium batteries of various shapes, such as a sheet battery of thinthickness and large area, or a card battery of a thin thickness and asmall area, are also desired. However, in the conventional technique ofenclosing a battery device comprised of positive and negative electrodesand an electrolytic solution within an exterior metallic can, it isdifficult to fabricate batteries of variable shapes such as thosedepicted above. Moreover, the use of the electrolytic solution tends tocomplicate the manufacturing process or renders it necessary to provideleakage-resistant means.

For overcoming the above-mentioned problems, such battery is beingenvisaged which uses a solid electrolyte employing an electricallyconductive organic high polymer material or organic ceramics, or agelated solid electrolyte having an electrolytic solution impregnatedinto a matrix polymer, referred to below as a gel electrolyte. With thesolid electrolyte battery, employing the solid electrolyte, or with thegel electrolyte battery, employing the gel electrolyte, in which theelectrolyte is immobilized, it is possible to fabricate the batteryusing a film-shaped exterior material to a reduced thickness, thusproviding a higher energy density than is possible with the conventionalbattery.

However, the gel electrolyte battery has a deficiency that, since theelectrolytic solution is held in the matrix polymer, the electrolyticsolution cannot sufficiently seep into the layer of the active materialsof the electrodes. As a result, lithium ions cannot be migratedsufficiently across the electrodes, with the result that a desiredbattery capacity cannot be achieved.

Moreover, in a gel electrolyte battery, the solvent in the gelelectrolyte tends to be decomposed at the time of activation charging toevolve gases, with the result that local gaps are produced between thelayer of the active material and the gel electrolyte to impair theadhesion between the layers of the active materials of the electrodesand the gel electrolyte. If such gap is produced between the layer ofthe active material and the gel electrolyte, the battery in storage isdeteriorated appreciably in battery voltage to prove a reject to lowerthe production yield. Moreover, lithium ion migration across theelectrodes is retarded to render it extremely difficult to realize thedesired battery capacity.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor the preparation of a non-aqueous electrolyte battery in whichadhesion between the layers of the active materials of the electrodesand the gel electrolyte is improved and kept to prevent voltage troublesfrom occurring to improve the production yield to enable the fabricationof a non-aqueous electrolyte battery having a high battery capacity.

In one aspect, the present invention provides a method for preparationof a gel electrolyte battery in which a battery device is accommodatedin an exterior material of a laminated film and sealed therein by heatfusion, in which the method includes a battery device preparation stepof layering a positive electrode and a negative electrode via a gelelectrolyte to form the battery device, an accommodating step ofaccommodating the battery device from the battery device preparationstep in the laminated film and a heating step of heating the batterydevice, accommodated in the laminated film in the accommodating step,under a pressured state.

In another aspect, the present invention provides a method forpreparation of a gel electrolyte battery in which a battery device isaccommodated in an exterior material of a laminated film and sealedtherein by heat fusion, in which the method includes a battery devicepreparation step of layering a positive electrode and a negativeelectrode via a gel electrolyte to form the battery device, anaccommodating step of accommodating the battery device from the batterydevice preparation step in the laminated film, a charging step ofcharging the battery device accommodated in the laminated film in theaccommodating step, a discharging step of discharging the battery devicefollowing the charging step and a heating step of heating the batterydevice from the discharging step under a pressured state.

According to the present invention, the seeping of a gel electrolyteinto the electrode is accelerated by applying pressuring and heating toa battery device provided with the gel electrolyte. Since the pressuringand heating are applied to the battery device, the adhesion between thegel electrolyte layer and a layer of the active material of theelectrode is restored to improve the production yield, even if a gap isproduced between the layer of the active material of the electrode andthe gel electrolyte due to gases evolved in activation charging. As aresult, the adhesion between the layer of the active material of theelectrode and the gel electrolyte is improved to provide a gelelectrolyte battery having a high capacity and superiorcharging/discharging characteristics and operational reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an illustrative structure of a gelelectrolyte battery according to the present invention.

FIG. 2 is a perspective view showing a battery device as it isaccommodated in an exterior film.

FIG. 3 is a cross-sectional view taken along line A–B in FIG. 2.

FIG. 4 is a perspective view showing the structure of a positiveelectrode.

FIG. 5 is a perspective view showing the structure of a negativeelectrode.

FIG. 6 is a graph showing the relation between the elapsed time and thevoltage for batteries of samples 24 and 25.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, preferred embodiments of the presentinvention will be explained in detail.

FIGS. 1 to 3 show an illustrative structure of a gel electrolyte batteryprepared in accordance with the present invention. Referring to FIG. 3,this gel electrolyte battery 1 includes a band-shaped positive electrode2, a band-shaped negative electrode 3 mounted facing the positiveelectrode 2, a gel electrolyte layer 4 formed on the positive electrode2 and the negative electrode 3, and a separator 5 arranged between thepositive electrode 2 carrying the gel electrolyte layer 4 and thenegative electrode 3 carrying the gel electrolyte layer 4.

In this gel electrolyte battery 1, the positive electrode 2 carrying thegel electrolyte layer 4, and the negative electrode 3 carrying the gelelectrolyte layer 4, are layered together with interposition of theseparator 5, and are coiled in the longitudinal direction to form abattery device 6, as shown in FIGS. 2 and 3. The battery device 6 soformed is hermetically sealed by an exterior film 7 of an insulatingmaterial. To the positive electrode 2 and the negative electrode 3 areconnected a positive electrode terminal 8 and a negative electrodeterminal 9, respectively, these terminals 8, 9 being fitted in anopening formed in a rim of an exterior film 7. A resin film 10 isarranged in an area of contact of the terminals 8, 9 with the exteriorfilm 7.

In the positive electrode 2, a positive active material layer 2 a,containing a positive active material is formed on each surface of thepositive electrode collector 2 b, as shown in FIG. 4. It is noted thatthe state in which the gel electrolyte layer 4 is formed on the positiveactive material layer 2 a is shown in FIG. 4.

As this positive electrode collector 2 b, a metal foil, such as an Alfoil, is used. This metal foil is preferably porous. By employing theporous metal foil as the positive electrode collector, it is possible toimprove adhesion strength between the positive electrode collector 2 band the positive active material layer 2 a. The porous metal foil usedmay be punching metal or expanded metal, but may also be a metal foil inwhich numerous openings are formed by etching.

The positive active material may be enumerated by metal oxides, metalsulfides, specified high molecular materials, or a lithium compoundoxide, having the general formula of Li_(x)MO₂, depending on the type ofthe batteries desired to be produced. In this general formula, M denotesone or more transition metals, and x is usually such that 0.05≦×≦1.12.

The transition metal M of the lithium compound oxide is preferably atleast one of cobalt (Co), nickel (Ni) and manganese (Mn). Specifiedexamples of the lithium compound oxides include LiCoO₂, LiNiO₂,Li_(x)Ni_(y)Co_(1-y)O₂, where x and y differ in magnitudes depending onthe charging/discharging state of the battery and usually 0<x<1 and0.7<y<1.0, or LiMn₂O₄.

These lithium compound oxides may be prepared using, as startingmaterials, lithium compounds and transition metal compounds, such ascarbonates, nitrates, sulfates, oxides, hydroxides or halogenides of thelithium transition metals. For example, the lithium compound oxides maybe prepared by metering lithium salt and transition metal startingmaterials, depending on the desired composition, sufficiently mixing thematerials and sintering, under heating, in an oxygen-containingatmosphere, in a temperature range of from 600° to 1000° C. There is noparticular limitation to the methods of mixing the respectivecomponents, such that the pulverulent salts may be directly mixed in adry state or may also be dissolved in water for mixing as an aqueoussolution.

As a binder contained in the positive active material layer 2 a, anysuitable known resin material, routinely used as a binder for thepositive active material layer of the non-aqueous electrolyte battery,may be used.

In the negative electrode 3, a negative active material layer 3 a,containing a negative active material, is formed on each surface of thenegative electrode collector 3 b, as shown in FIG. 5. It is noted thatthe state in which the gel electrolyte layer 4 has been formed on thenegative active material layer 3 a is shown in FIG. 5.

As this negative electrode collector 3 b, metal foils, such as a copperor nickel foil, is used. This metal foil is preferably porous. Byemploying a porous metal foil as a negative electrode collector, it ispossible to improve adhesion strength between the negative electrodecollector 3 b and the negative active material layer 3 a. The porousmetal foil used may be punching metal or expanded metal, but may also bea metal foil in which numerous openings are formed by etching.

As the negative active material, such materials capable ofdoping/undoping lithium may be used. These materials, capable ofdoping/undoping lithium, may be enumerated by lithium metals, lithiummetal alloys and carbon materials. Examples of the carbon materialsinclude natural or synthetic graphite, pyrolytic carbon, cokes, such aspitch coke, needle coke or petroleum coke, carbon blacks, such asacetylene black, vitreous carbon, activated charcoal, carbon fibers,sintered organic high molecular materials, such as cellulose, phenolicresins or furan resins, fired at suitable temperatures, and carbonfibers.

As binders contained in the negative active material layer 3 a, anysuitable known resin materials, routinely used as a binder for thenegative active material layer of the non-aqueous electrolyte battery,may be used.

In the gel electrolyte layer 4, the non-aqueous electrolyte, comprisedof an electrolytic salt dissolved in a non-aqueous solvent, is gelatedby a matrix polymer.

As the electrolyte salt, LiPF₆, LiClO₄, LiCF₃SO₃, LiAsF₆, LiBF₄,LiN(CF₃SO₃)₂ or C₄F₉SO₃Li, may be used alone or in combination. Ofthese, LiPF₆ is preferred in view of ionic conductivity. Meanwhile, theelectrolyte salt is preferably formulated to a concentration of 0.10mol/l to 2.0 mol/l, based on a non-aqueous solvent, in order to givesatisfactory ionic conductivity.

There is no particular limitation to the chemical structure of thematrix polymer, if the polymer per se or the gel electrolyte employingthis polymer per se exhibits ionic conductivity not lower than 1 mS/cmat ambient temperature. As this matrix polymer, polyacrylonitrile,polyacrylonitrile copolymers, polyoxylene oxides or polyoxylene oxidecopolymers, may be used. The vinyl copolymer monomers may be enumeratedby, for example, hexafluoropropylene, tetrafluoroethylene, vinylacetate, methyl methacrylate, butyl methacrylate, methyl acrylate, butylacrylate, itaconic acid, hydrogenated methyl acrylate, hydrogenatedethyl acrylate, acrylic amide, vinyl chloride, vinylidene fluoride, andvinylidene chloride. In addition, acrylonitrile-butadiene rubber,acrylonitrile-butadiene styrene resin, acrylonitrile-polyethylenechloride propylene diene styrenic resin, acrylonitrile-vinyl chlorideresin, acrylonitrile-methacrylate resin and acrylonitrile-acrylateresin, may also be used. Moreover, polyether modified siloxane orcopolymers thereof may be used. The above-mentioned matrix polymer maybe used singly or in combination.

As the non-aqueous solvent, ethylene carbonate, propylene carbonate,butylene carbonate, ã-butyrolactone, 2,4-difluoroanisole,2,6-difluoroanisole or 4-bormovelatol, may be used singly or incombination.

If a multi-layer film, such as exterior film 7, is used as an exteriormaterial for a battery, it is preferred to use solvents boiling at atemperature 150° C. or higher, such as ethylene carbonate, propylenecarbonate, ã-butyrolactone, 2,4-difluoroanisole, 2,6-difluoroanisole or4-bormovelatol, in combination.

Moreover, in the gel electrolyte battery, embodying the presentinvention, the content of the low boiling solvent in the non-aqueoussolvent is set to 1 wt % or less. Meanwhile, the low boiling solventmeans such a solvent boiling at 110° C. or less. If the content of thelow boiling solvent exceeds 1 wt %, the low boiling solvent content isvolatilized off in preparing the gel electrolyte battery by processingthe battery device with heating, thereby swelling the exterior film todeform the battery shape.

Such low boiling solvent may specifically be enumerated byã-valerolactone, diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, methyl propionate,dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

As the separator 5, a micro-porous thin film, mainly composed ofpolyolefin, such as polypropylene, polyethylene or compound materialsthereof, may be used. It is more preferred to use a micro-porous thinfilm, improved in wettability with respect to the electrolytic solutionby exploiting surfactants or corona discharge processing. This preventsthe internal resistance of the battery from increasing.

Although there is no limitation to the porosity of the separator 5, itis preferred to set the porosity to 30 to 80%. If the porosity of theseparator 5 is less than 30%, output characteristics of the battery arelowered significantly. If the porosity of the separator 5 exceeds 60%,the separator 5 is lowered in mechanical strength. Moreover, there is nolimitation to the diameter of the pores or to the thickness of theseparator 5, however, the pore diameter is preferably set to 1 μm orless to prevent internal shorting and to display shutdown effects bypore closure.

The thickness of the separator 5 is preferably on the order of 5 to 35μm. Moreover, if the relation between the mechanical strength and theelectrical resistance of the separator 5 is taken into account, thethickness of the separator 5 is preferably on the order of 7 to 20 μm.

It suffices if the exterior film 7 exhibits moisture-proofness. Forexample, a three-layer film comprised of a nylon film, an Al foil and apolyethylene film, layered in this order and stuck together, may beused.

In the gel electrolyte battery 1, prepared in accordance with thepresent invention, in which the adhesion between the gel electrolyte andthe active material layers of the electrodes is improved by pressuringand heating of the battery device 6, occurrence of voltage troubles maybe lowered, while the production yield is improved. Moreover, the gelelectrolyte battery 1 has a high battery capacity and superior batterycharacteristics.

The gel electrolyte battery 1, embodying the present invention, asdiscussed above, is prepared in the following manner.

For preparing the positive electrode 2, powders of a positive activematerial, an electrifying agent, such as carbon black or graphite, and abinder, such as polyvinylidene fluoride, are mixed uniformly, and addedto with a solvent, such as dimethyl formamide or n-methyl pyrrolidone,to yield a paste-like positive electrode mixture, which then is evenlycoated on a metal foil, such as an Al foil, operating as the positiveelectrode collector 2 b. The resulting assembly then is dried to form apositive electrode sheet carrying the positive active material layer 2 athereon. The above-mentioned positive electrode mixture may be added towith e.g., known additives.

On the positive active material layer 2 a of the positive electrodesheet is formed the gel electrolyte layer 4. For preparing the gelelectrolyte layer 4, an electrolyte salt is dissolved in the non-aqueoussolvent to prepare a non-aqueous solution. To this non-aqueous solutionis added a matrix polymer. The resulting mass is stirred thoroughly todissolve the matrix polymer to produce a sol-like electrolytic solution28.

On this positive electrode 2 is applied a pre-set amount of theelectrolytic solution. The resulting mass is cooled at room temperatureto permit the matrix polymer to be gelated to form the gel electrolytelayer 4 on the positive active material layer 2 a.

A positive electrode sheet, carrying the gel electrolyte layer 4, issliced in a strip shape. To a portion of the positive electrodecollector 2 b not carrying the positive active material layer 2 a iswelded a lead wire of e.g., aluminum, to yield a positive electrodeterminal 8. This gives a strip-like positive electrode 2 carrying thegel electrolyte layer 4 thereon.

In preparing the negative electrode 3, powders of the negative activematerial and a binder such as polyvinylidene fluoride are mixed evenlyand added to with a solvent such as dimethyl formaldehyde or n-methylpyrrolidone to prepare a paste-like negative electrode mixture. Thisnegative electrode mixture is evenly coated on a metal foil, such ascopper foil or nickel foil operating as the negative electrode collector3 b, and dried to form a negative electrode sheet carrying the negativeactive material layer 3 a thereon. This negative electrode mixture maybe added to with e.g., a known additive.

On the negative electrode collector 3 b of the negative electrode sheetis formed the gel electrolyte layer 4. For forming the gel electrolytelayer 4, a pre-set amount of the electrolytic solution, prepared asdiscussed above, is coated on the layer of the negative active material.The resulting mass is allowed to cool at room temperature to permit thematrix polymer to be gelated to form the gel electrolyte layer 4 on thenegative electrode collector 3 b.

A negative electrode sheet, carrying the gel electrolyte layer 4, issliced in a strip shape. To a portion of the negative electrodecollector 3 b not carrying the negative active material layer 3 a iswelded a lead wire of e.g., nickel, to yield a negative electrodeterminal 9. This gives a strip-like negative electrode 3 carrying thegel electrolyte layer 4 thereon.

The positive electrode 2 and the negative electrode 3, prepared asdiscussed above, are layered together, with the sides thereof carryingthe gel electrolyte layers 4 facing each other, and with the separator 5arranged between the positive electrode 2 and the negative electrode 3,to form a layered electrode assembly, which then is coiledlongitudinally to form the battery device 6.

Finally, ths battery device 6 is enclosed in the exterior film 7 of aninsulating material, and a resin film 10 is placed in an overlappingportion of the positive electrode terminal 8, negative electrodeterminal 9 and the exterior film 7. The outer rim of the exterior film 7is sealed, the positive electrode terminal 8 and the negative electrodeterminal 9 are engaged in the sealing portion of the exterior film 7.The terminals 8, 9 are led to outside, as the battery device 6 istightly sealed in the exterior film 7.

When the battery device 6 is packed in the exterior film 7, the resinfilm 10 is placed at a contacting portion of the exterior film 7,positive electrode terminal 8 and the negative electrode terminal 9 toprevent shorting of the exterior film 7 due to burrs as well as toimprove the adhesion between the exterior film 7, positive electrodeterminal 8 and the negative electrode terminal 9.

There is no limitation to the material of the resin film 10 if itexhibits adhesion characteristics to the positive electrode terminal 8and to the negative electrode terminal 9. It is however preferred to usesuch material which is composed only of a polyolefin resin, such aspolyethylene, polypropylene, modified polyethylene, modifiedpolypropylene or copolymers thereof. The thickness of the resin film 10is preferably 20 to 200 μm in terms of a thickness prior to thermalfusion. If the thickness of the resin film 10 is less than 20 μm,tractability is lowered. If it is thicker than 300 μm, it is permeableto water to render it difficult to keep the inside of the batteryair-tight.

According to the present invention, the battery device 6, packed by theexterior film 7, is pressed from above and below, whilst the batterydevice 6 is subjected to heat treatment. By pressuring and heat-treatingthe battery device 6, packed by the exterior film 7, the non-aqueouselectrolytic solution, held by the matrix polymer in the gelelectrolyte, can be made to seep effectively into the active materiallayers. With the non-aqueous electrolytic solution seeping into theactive material layers, it is possible to improve adhesion between thegel electrolyte layer 4 and the active material layers to provide thegel electrolyte battery 1 having a high capacity and superiorcharging/discharging characteristics.

In particular, if the positive electrode 2 carrying the gel electrolytelayer 4 and the negative electrode 3 also carrying the gel electrolytelayer 4 are superposed together with the separator 5 in-between, theelectrolyte can be permeated sufficiently into the separator pores, thusintensifying the favorable effect.

In this case, the heating temperature for the battery device 6 ispreferably not lower than the sol-gel transition temperature of the gelelectrolyte. The reason is that fluidity is higher in the sol state thanin the gel state such that the non-aqueous electrolytic solution in thegel electrolyte layer is more liable. to seep into the active materiallayers to shorten the heating/pressuring time.

Specifically, the heating temperature for the battery device 6 ispreferably 50° C. to 105° C., and more preferably 70° C. to 100° C. Withthe heating temperature higher than 105° C. part of the non-aqueoussolvent in the gel electrolyte undergoes pyrolysis so that the desiredbattery capacity cannot be achieved. If the heating temperature is lowerthan 50° C. the non-aqueous solvent can seep into the non-aqueouselectrolytic solution in the active material layers only insufficiently,while it takes much time for the non-aqueous electrolytic solution toseep into the active material layers to lower the production efficiency.

The pressure applied to the battery device 6 is preferably in a rangefrom 490 kPa to 2450 kPa. If the pressure is lower than 490 kPa, theadhesion between the gel electrolyte layer 4 and the active materiallayers is insufficient such that desired charging/dischargingcharacteristics cannot be achieved. If the pressure is higher than 2450kPa, the gel electrolyte layer 4 is collapsed and destroyed.

The battery device 6 packed in the exterior film 7 is pressured andheated as the exterior film 7 is sandwiched by a metallic heat blockfrom above and below. In this case, the exterior film 7 is not directlysandwiched by the heat block. It is more preferred that a block of aheat-resistant rubber is arranged between the exterior film 7 and theheat block such that the battery device 6 in the exterior film 7 ispressured and heated through this rubber block. If the battery device 6is pressured and heated as the exterior film 7 is directly sandwiched bythe heat block, the temperature of the heat block is difficult to holdto render the temperature unstable. Moreover, it is difficult topressure the battery device 6 uniformly. By using the block of theheat-resistant rubber, it is possible to provide for uniform and stablepressuring and heating of the battery device 6.

For activating the gel electrolyte battery 1, prepared as discussedabove, it is charged under a pre-set charging condition, and the batteryso charged is discharged under a pre-set discharging condition. Thecharging/discharging conditions may be suitably set depending e.g., onthe battery shape or size.

If a gas is yielded during the activation charging, due to solventdecomposition in the gel electrolyte, there is produced a local gapbetween the active material layers and the gel electrolyte layer 4, suchthat adhesion between the active material layers and the gel electrolytelayer 4 is lost in this area to deteriorate the battery voltage duringstorage to increase the rate of rejects thus lowering the productionyield. Moreover, lithium ions cannot be migrated sufficiently such thatthe desired battery capacity cannot be achieved.

So, with the present invention, the gel electrolyte battery 1 from theactivation charging and discharging process is pressured from above andbelow, while the battery device is heated. By pressuring the gelelectrolyte battery 1, packed in the exterior film 7, and by heating thebattery body, tight adhesion between the gel electrolyte and theelectrode can be achieved even if a gap is produced between the gelelectrolyte layer 4 and the active material layers due to gas evolutionin the course of the activation charging process. By maintaining thetight adhesion between the gel electrolyte layer 4 and the activematerial layers, the gel electrolyte battery 1 produced may be free ofvoltage defects, while being of high capacity and superiorcharging/discharging characteristics.

The heating temperature for the gel electrolyte battery 1 at this timeis preferably not less than the sol-gel transition temperature of thesol electrolyte. Specifically, the heating temperature for the batterydevice is preferably 50° to 105° C. and more preferably 70° to 100° C.if the heating temperature is higher than 105° C., part of thenon-aqueous solvent in the gel electrolyte undergoes pyrolysis so thatthe desired battery capacity cannot be achieved. If the heatingtemperature is lower than 50° C., the non-aqueous solvent can seep intothe non-aqueous electrolytic solution in the active material layers onlyinsufficiently, while it takes much time for the non-aqueouselectrolytic solution to seep into the active material layers to lowerthe production efficiency.

The pressure applied to the battery device 6 is preferably in a rangefrom 490 kPa to 2450 kPa. If the pressure is lower than 490 kPa, theadhesion between the gel electrolyte layer 4 and the active materiallayers is insufficient such that desired charging/dischargingcharacteristics cannot be achieved. If the pressure is higher than 2450kPa, the gel electrolyte layer 4 is collapsed and destroyed.

The battery device 6 packed in the exterior film 7 is pressured andheated as the exterior film 7 is sandwiched by a metallic heat blockfrom above and below. In this case, the exterior film 7 is not directlysandwiched by the heat block. It is more preferred that a block of aheat-resistant rubber is arranged between the exterior film 7 and theheat block such that the battery device 6 in the exterior film 7 ispressured and heated through this rubber block. If the battery device 6is pressured and heated as the exterior film 7 is directly sandwiched bythe heat block, the temperature of the heat block is difficult to holdto render the temperature unstable. Moreover, it is difficult topressure the battery device 6 uniformly. By using the block of theheat-resistant rubber, it is possible to provide for uniform and stablepressuring and heating of the battery device 6.

The gel electrolyte battery 1, prepared in accordance with the presentinvention, is improved in tight adhesion between the gel electrolytelayer 4 and the active material layers, and hence is of a high capacityand superior in charging/discharging characteristics.

If a gap is produced between the gel electrolyte layer and the activematerial layers, due to the gas evolved during the activation chargingprocess, it is possible to fill the gap to maintain tight adhesionbetween the electrolyte layer and the active material layers to increasethe production yield. With the gel electrolyte battery 1, prepared asdiscussed above, lithium migration across the positive and negativeelectrodes occurs smoothly, thus assuring the high capacity and thesuperior charging/discharging characteristics. Moreover, the gelelectrolyte battery 1, manufactured in accordance with the presentinvention, is superior in operational reliability under high temperatureconditions.

In the above-described embodiment, the strip-shaped positive electrode 2and the equally strip-shaped negative electrode 3 are layered togetherand the resulting assembly is coiled longitudinally to produce thebattery device 6. The present invention, however, is not limited to thisconfiguration and may be applied to such a case wherein a rectangularpositive electrode 2 is layered together with a rectangular negativeelectrode 3 to provide a layered electrode, or to such a case whereinlayered electrodes are collapsed alternately.

The gel electrolyte battery 1 embodying the present invention is notlimited to its shape, while it may be of any suitable size, such as thintype or a large size. Moreover, the present invention is applicable bothto a primary battery and a secondary battery,

EXAMPLES

The batteries were prepared as indicated below to check the meritoriouseffect of the present invention.

Example

First, a positive electrode was prepared as now explained. First, 91parts by weight of LiCoO2, with a mean particle size of 5 μm, as apositive active material, 6 parts by weight of carbon black, as anelectrifying agent, and 3 parts by weight of polyvinylidene fluoride,were mixed together to give a positive electrode mixture. This positiveelectrode mixture was then dispersed in N-methyl pyrrolidone, as asolvent, to form a paste.

The positive electrode mixture paste was then coated evenly on bothsurfaces of a strip-like aluminum foil, operating as a positiveelectrode collector, with a thickness of 20 μm, and the resulting masswas dried. The dried mass was compression-molded by a roll press to forma positive active material layer. An aluminum lead was welded to aportion of the positive electrode collector not carrying the positiveactive material layer to form a positive electrode terminal to completea positive electrode.

A negative electrode was prepared in the following manner. First, 90parts by weight of graphite, with a mean particle size of 20 μm, as anegative active material, and 10 parts by weight of polyvinylidenefluoride, as a binder, were mixed together to a negative electrodemixture. This negative electrode mixture was dispersed in N-methylpyrrolidone as a solvent to produce a paste.

The resulting negative electrode mixture paste was uniformly coated onboth surfaces of the strip-like copper foil, as a negative electrodecollector, and dried. The dried mass was compression-molded by a rollpress to form a negative active material layer. A copper lead was weldedto a portion of the negative electrode collector not carrying thenegative active material layer to form a negative electrode terminal tocomplete a negative electrode.

A gel electrolyte layer was then formed on each of the positiveelectrode and the negative electrode as follows:

In a mixed solvent, containing ethylene carbonate, propylene carbonateand ã-butyrolactone at a weight ratio of 5:3:2, LiPF₆ was dissolved at aconcentration of 1.2 mol/l to prepare a non-aqueous electrolyticsolution. Into this non-aqueous electrolytic solution was then added acopolymer of vinylidene fluoride and hexafluoropropylene, in an amountof 15 wt %, to produce a high molecular electrolytic solution.

The resulting electrolytic solution was then uniformly coated on bothsurfaces of the positive and negative electrodes by a doctor blademethod. The electrolytic solution then was gelated to form a gelelectrolyte layer on each of the surfaces of the positive and negativeelectrodes.

A battery then was assembled as follows: the strip-like positiveelectrode, carrying the gel electrolyte layers on its both surfaces, andthe strip-like negative electrode, similarly carrying the gelelectrolyte layers on its both surfaces, were layered together withinterposition of a separator, to form a layered mass, which then wascoiled longitudinally to from a battery device. The separator used was aporous polyethylene film.

This battery device then was sandwiched by an moisture-proofing exteriorfilm, comprised of a nylon layer 25 μm thick, an Al layer 40 μm thickand a polypropylene film 30 μm thick, layered together. An outer rim ofthe exterior film was heat-fused under reduced pressure and sealed totightly seal the battery device in the exterior film. At this time, thepositive and negative electrode terminals were sandwiched in the openingpart of the exterior film and a polyolefin film was arranged in acontact portion of the exterior film, the positive electrode terminaland the negative electrode terminal.

The battery device, thus sealed in the exterior film, was then pressuredat a pre-set pressure and was subjected to a first pressuring/heatingprocessing at a pre-set temperature to complete a gel electrolytebattery 53 mm ×34 mm in size and 3 mm in thickness. The first pressuringand heating processing was carried out as the battery device sealed bythe exterior film was clenched by a pair of heating blocks of siliconrubber.

Then, gel electrolyte batteries of samples 1 to 18 were prepared by thesame method as discussed above except changing the pressure applied tothe battery device sealed in the exterior film and the heatingprocessing temperature to the battery device as indicated in Table 1.Meanwhile, the sample 18 of the battery device was pressured and heatedwithout using a heating block of silicon rubber.

The batteries of the samples 1 to 18 were then tested as tocharging/discharging. First, a constant current charging at 500 mA wascarried out. When the battery voltage reached 4.2 V, the constantcurrent charging was changed over to constant voltage charging andcharging was carried out for 2.5 hours as the voltage of 4.2 V was kept.The constant current discharging then was carried out at 100 mA andfinished when the battery voltage fell to 3.0 V.

The discharging capacity was measured at the first cycle time. With thedischarging capacity of the sample 4 battery set to 100, the dischargingcapacity ratio (%) with respect to the battery of the sample 4 was foundfor each battery.

After repeating the above-mentioned charging/discharging process for tencycles, the batteries were disintegrated and the appearance of thenegative electrode surface was checked visually using a microscope. Iflithium precipitation was noticed on the negative electrode surface of agiven sample, lithium precipitation was determined to have occurred inthat sample. If no lithium precipitation was noticed on the negativeelectrode surface of a given sample, lithium precipitation wasdetermined not to have occurred in that sample.

For the samples 1 to 18, the discharge capacity ratio and observedresults on the negative electrode surface are shown in Table 1 alongwith the pressure and heating processing temperature as applied to thebattery device samples.

TABLE 1 heating discharge Li detection temperature pressure capacity onnegative (° C.) (kPa) ratio (%) electrode surface note sample 1 90 29485 not detected sample 2 90 490 95 not detected sample 3 90 980 99 notdetected sample 4 90 1470 100 not detected sample 5 90 1960 99 notdetected sample 6 90 2450 98 not detected sample 7 90 2646 97 detectedinternal shorting occurred sample 8 45 1470 85 detected sample 9 50 147093 not detected sample 10 60 1470 95 not detected sample 11 70 1470 97not detected sample 12 80 1470 98.5 not detected sample 13 100 1470 99.5not detected sample 14 105 1470 98.5 not detected sample 15 110 1470 97not detected swelling occurred sample 16 — — 79 detected sample 17 95 —83 detected sample 18 90 1470 95 detected

As also shown in Table 1, adhesion between the gel electrolyte layer andthe active material layer was not maintained with the battery of sample16 for which no measures have been taken for the battery device, suchthat the discharge capacity was lowered. Also, lithium precipitation wasnoticed on the negative electrode.

For the battery device of sample 17, for which heating was made butpressuring was not made, adhesion was not improved between the gelelectrolyte layer and the active material layer, such that a desireddischarge capacity was not produced. Also, lithium precipitation wasnoticed on the negative electrode.

On the other hand, with the battery samples 1 to 15, processed withheating and pressuring, adhesion between the gel electrolyte layer andthe active material layer could be improved, while sufficient dischargecapacity was obtained. Moreover, in many cases, lithium precipitation onthe negative electrode was not noticed.

As for the heating temperature, seeping of the electrolytic solutioninto the active material layer was not sufficient for the sample 8 withthe heating temperature of 45° C. Also, lithium precipitation on thenegative electrode was noticed. With the sample 15, with the heatingtemperature of 110° C., the exterior film 7 was seen to have swollen,possibly due to partial volatilization of the non-aqueous solvent in thegel electrolyte.

As for the pressure, adhesion between the gel electrolyte layer and theactive material layer was not sufficient for the sample 1 with thepressure of 294 kPa (3 kgf/cm²), such that desired charging/dischargingcharacteristics were not achieved. On the other hand, with the sample 7with the pressure of 2646 kPa (27 kgf/cm²), the gel electrolyte wascollapsed and destroyed, whilst internal shorting was noticed andlithium precipitation was seen on the negative electrode.

The battery devices of the samples 2 to 6 and 9 to 14, in which theheating processing temperature for the battery device is 50° C. to 105°C. and the pressure applied to the battery device is 490 kPa (5 kgf/cm²)to 2450 kPa (25 kgf/cm²), exhibited high discharging capacity, whilethere was no lithium precipitation on the negative electrode.

So, with the heating temperature for the battery device in a range from50 to 105° C. and the pressure applied to the battery device in a rangefrom 490 kPa (5 kgf/cm²) to 2450 kPa (25 kgf/cm²), permeation of the gelelectrolyte into the active material layer is accelerated to improve theadhesion between the active material layer and the gel electrolytelayer. The battery thus fabricated has been shown to exhibit superiorcharging/discharging characteristics, with smooth lithium migrationacross the positive and negative electrodes.

With the sample 18, in which the laminated film is directly clenched bythe heating block, without interposition of the silicon rubber block, inapplying the pressuring and heating to the battery device, it has beenshown that, in the absence of the silicon rubber block, uniformpressuring and heating cannot be applied to the battery device, suchthat the adhesion between the gel electrolyte layer and the activematerial layer is insufficient, thus permitting lithium to beprecipitated on the negative electrode.

So, it has been shown that, by the interposition of the silicon ruberblock between the heating block and the laminated film, pressuring andheating can be applied uniformly in stability to the battery device toraise the adhesion between the gel electrolyte layer and the activematerial layer.

In samples 19 to 23, now explained, batteries were prepared as thenon-aqueous solvent components contained in the gel electrolyte werevaried. Specifically, the gel electrolyte batteries of the samples 19 to23 were prepared in the same way as described above except setting theamount of dimethoxyethane (DME) as a low boiling solvent contained inthe non-aqueous solvent making up the gel electrolyte layer as shown inTable 2. Meanwhile, the pressure applied to the battery device was 1470kPa (15 kgf/cm²), with the heating processing temperature being set to90° C.

The charging/discharging test was conducted on the battery devices ofthe samples 19 to 23, thus prepared, in the same testing method asdiscussed above. The discharge capacity ratio and observed results onthe negative electrode surface of the batteries of the samples 19 to 23are shown in Table 2, along with the pressure and heating temperatureapplied to the battery devices and the content of DME in theelectrolytic solution.

TABLE 2 DME discharge Li detection on content capacity the negative (wt%) ratio (%) electrode surface note sample 19 0.1 100 not detectedsample 20 0.5 98 not detected sample 21 1.0 93 not detected sample 221.2 85 not detected swelling occurred sample 23 1.5 70 detected swellingoccurred

As may be seen from Table 2, with the samples 22, 23, in which theamount of the low boiling solvent was set to 1.2 wt % and to 1.5 wt %,respectively, part of the solvent was volatilized off in pressuring andheating the battery device, resulting in swollen exterior films. Withthe sample 23, lithium was precipitated on the negative electrode.

On the other hand, with the samples 19 to 21 in which the amount of thelow boiling solvent was set to 1.0 wt % or less, the battery shape couldbe maintained, without partial solvent volatilization.

So, it was found that, by setting the content of the low boiling solventin the gel electrolyte to 1 wt % or less, the exterior film 7 could beprevented from becoming swollen due to volatilization of the low boilingsolvent component.

A gel electrolyte battery (sample 24) obtained on processing the sample4 (gel electrolyte battery in which the heating processing temperatureto the battery device is set to 90° C. and the pressuring pressure tothe battery device was set to 1470 kPa) with activation charging anddischarging, followed by pressuring at 1470 kPa and by second heatingprocessing at 90° C., is hereinafter explained. Meanwhile, the secondpressuring and heating for the gel electrolyte battery was performed asthe gel electrolyte battery was clenched between a pair of heat blocksvia silicon rubber blocks.

As sample 25, a gel electrolyte battery was prepared in the same way assample 24, except not applying second pressuring and heating to the gelelectrolyte battery processed with activation charging and discharging.

Also, as sample 26, a gel electrolyte battery was prepared in the sameway as sample 24, except that, instead of applying pressuring andheating using the heat blocks to the battery device, the battery deviceis kept in a constant temperature vessel for solely performing heatingwithout pressuring in the first pressuring and heating processing.

First, for 100 sample-24 batteries and 100 sample-25 batteries, theyield was evaluated. In evaluating the yield, changes with lapse of timeof the open-circuit voltage (OCV) directly as from battery completionwere measured and, accepting the batteries which maintained pre-setvoltage after lapse of a pre-set time, the rate of the acceptedbatteries in 100 batteries was checked. FIG. 6 shows the relationbetween the time elapsed (in days) and the OCV (V) for the batteries ofthe samples 24 and 25.

As may be seen from FIG. 6, with the battery device of the sample 25,corresponding to the gel electrolyte battery processed with activationcharging and discharging but not processed with second pressuring andheating, there was produced a local gap between the gel electrolytelayer and the active material layer such that the adhesion between thegel electrolyte layer and the active material layer could not be raised,with the voltage being lowered significantly with lapse of time, withthe yield amounting to only 92%.

On the other hand, the battery of sample 24, corresponding to the gelelectrolyte battery processed with activation charging and dischargingand also with second pressuring and heating, there was produced no localgap between the gel electrolyte layer and the active material layer,such that adhesion between the gel electrolyte layer and the activematerial layer could be maintained, with the voltage lowering with lapseof time being smaller by approximately 50 mV than in the battery deviceof sample 25, with the yield being as high as 96%.

Next, the battery devices of the samples 24 and 25 were put tocharging/discharging tests under pre-set charging/discharging conditionsto measure the discharging capacity. It was found that, with the batteryof sample 24, the discharging capacity was improved by approximately 5%on an average as compared to the battery of sample 25, thus producing asufficient discharging capacity.

Thus, if a gap is produced between the active material layer and the gelelectrolyte layer due to gas evolution in activation charging, this gapcan be filled by the pressuring and heating of the gel electrolytebattery after the charging/discharging, thereby improving the adhesionbetween the active material layer and the gel electrolyte layer forassuring a high battery yield. It was also found that, with the gelelectrolyte battery, so prepared, lithium migration across the positiveand negative electrodes occurs smoothly to suppress the voltage loweringto display superior charging/discharging characteristics.

Moreover, with the batteries of the samples 24 and 25, heatingcharacteristics in the high voltage charging state were evaluated. Inevaluating the heating characteristics, as the battery devices of thesamples 24 and 25 were charged to 4.25 V or to 4.40V, the batteries wereheated to 135° C., 140° C., 145° C., 150° C. and to 155° C., and thebatteries in which the battery function was lost were denoted NG, whilstthose which kept the battery functions were denoted OK. The results areshown in Table 3.

TABLE 3 voltage (V) 135° C. 140° C. 145° C. 150° C. 155° C. sample 4.25OK OK OK OK OK 24 4.40 OK OK NG — — sample 4.25 OK OK NG — — 25 4.40 OKNG — — —

It may be seen from the above Table 3 that, with the battery of thesample 24, in which a gel electrolyte battery following activationcharging and discharging is processed with the second heating andpressuring, the battery function is maintained up to 150° C. for 4.25Vcharging and up to 140° C. for 4.40 V charging, whereas, with thebattery of the sample 25, not processed with the second heating andpressuring, the battery function is maintained only up to 140° C. for4.25 V charging and up to 135° C. for 4.40 V charging. It may be seenfrom this that, by subjecting the gel electrolyte battery following theactivation charging and discharging to the second heating andpressuring, high reliability may be maintained in the resulting batteryeven at elevated temperatures.

The battery capacities of the sample 24 and 26 batteries were comparedto one another.

In the sample 24 battery obtained on subjecting a battery device tofirst pressuring and heating, the force of fusion between the separator,gel electrolyte layer and the active material layer is strong. It isnoted that, at this time point, the sample 24 is equivalent to thesample 4. The result is that, if the gas is evolved at the time ofactivation charging, adhesion between the positive and negativeelectrodes is substantially maintained, thus yielding a high capacity.

If then the battery is subjected to the second pressuring and heating(sample 24), the gap between the electrodes, produced due to gasevolution, is filled by fusion, thus giving a higher capacity (with thedischarge capacity ratio to the discharge capacity of the sample 4 being101%).

On the other hand, in the sample 26 battery, obtained on subjecting abattery device only to heating processing without applying the pressureto the battery device, with the sample 26 battery being equivalent atthis time point to the sample 17 battery, the force of fusion betweenthe separator, gel electrolyte layer and the active material layer isweak. So, the positive and negative electrodes are separated away fromeach other by the gas evolved at the time of activation charging andhence lithium ions are not intercalated to the negative electrode withthe result that no sufficient capacity is obtained. For example, thedischarge capacity ratio to the charging capacity of the sample 4 is83%.

By subjecting the battery to the second pressuring and heating followingdischarging (sample 26), the spacing between the electrodes, separatedfrom each other by gas evolution, are fused together. Since thisintercalates the lithium ions to the negative electrode afterre-charging, the capacity is increased. The discharge capacity ratio tothe charging capacity of the sample 4 is 98%.

The effect in capacity increase brought about by the second pressuringand heating of the gel electrolyte battery following the activationcharging is more outstanding in the case of the battery of the sample 26not initially processed with the first pressuring and heating. With thesample 24 battery which initially performs first pressuring and heating,the adhesion between the electrodes is inherently strong so that thedegree of capacity increase is small.

However, since in general the first pressuring and heating at the outsetis more liable to give a gel electrolyte battery of high capacity,heating and pressuring the battery device or the gel electrolyte batterytwice before and after the activation charging and discharging may besaid to be more effective.

1. A method for preparation of a gel electrolyte battery in which abattery device is accommodated in an exterior material, said exteriormaterial comprising a laminated film and sealed therein by heat fusion,said method comprising: (a) a battery device preparation step oflayering a positive electrode, a negative electrode and a gelelectrolyte to form said battery device; (b) an accommodating step ofaccommodating the battery device from said battery device preparationstep (a) within said laminated film and sealing said laminated film; (c)a first heating step of heating said battery device sealed in saidlaminated film under a pressured state; (d) a charging step of chargingthe battery device accommodated in said laminated film in saidaccommodating step (b), wherein said step (d) occurs after step (c); (e)a discharging step of discharging the battery device following thecharging step (d); and (f) a second heating step of heating said batterydevice under a pressured state, said second heating step (f) occurringafter said discharging step (e).
 2. The method for preparation of a gelelectrolyte battery according to claim 1 wherein, in the second heatingstep (f) the pressure applied to the battery device is at least 490 andat most 2450 kPa.
 3. The method for preparation of a gel electrolytebattery according to claim 1 wherein, in the second heating step (f) thetemperature of heating the battery device is at least 50° C. to at most105° C.
 4. The method for preparation of a gel electrolyte batteryaccording to claim 1 wherein, in the second heating step (f) thepressuring and heating are applied through a resin of heat-resistantrubber.
 5. The method for preparation of a gel electrolyte batteryaccording to claim 4 wherein the heat-resistant rubber is siliconrubber.
 6. The method for preparation of a gel electrolyte batteryaccording to claim 1 wherein the exterior material comprises a laminatedfilm, said laminated film comprising an Al foil and resin layers on bothsides of said Al foil.
 7. The method for preparation of a gelelectrolyte battery according to claim 1 wherein, in said battery devicepreparation step (a), the gel electrolyte comprises a matrix polymer, anon-aqueous solvent and an electrolyte salt, and wherein a ratio B/A isat most 1 wt %, B being of the amount of the non-aqueous solvent boilingat a temperature of at most 110° C. under ambient pressure, A being thetotal amount of the non-aqueous solvent contained in the gelelectrolyte.
 8. The method for preparation of a gel electrolyte batteryaccording to claim 1 wherein, in said battery device preparation step(a), the matrix polymer in a gel electrolyte comprises at least onematerial selected from the group of polyacrylonitrile, polyethyleneoxides, hexafluoropropylene, tetrafluoroethylene, vinyl acetate, methylmethacrylate, butyl methacrylate, methyl acrylate, butyl acrylate,itaconic acid, hydrogenated methyl acrylate, hydrogenated ethylacrylate, acrylic amide, vinyl chloride, vinylidene fluoride, vinylidenechloride, acrylonitrile-butadiene rubber, acrylonitrile-butadienestyrene resin, acrylonitrile-polyethylene chloride propylene dienestyrenic resin, acrylonitrile-vinyl chloride resin,acrylonitrile-methacrylate resin, acrylonitrile-acrylate resin,polyether modified siloxane and copolymers thereof.
 9. The method forpreparation of a gel electrolyte battery according to claim 1 wherein,in said battery device preparation step (a), a polyolefinic micro-porousseparator is arranged, along with the gel electrolyte, between thepositive and negative electrodes.
 10. The method for preparation of agel electrolyte battery according to claim 1 wherein, in said batterydevice preparation step (a), a strip-like positive electrode and astrip-like negative electrode are layered together via a gel electrolyteand coiled longitudinally to form a battery device.
 11. The method forpreparation of a gel electrolyte battery according to claim 10 wherein,in said battery device preparation step (a), a micro-porous separator isarranged between the strip-like positive electrode made up of thepositive active material layer and a gel electrolyte layer formedthereon and the strip-like negative electrode made up of the negativeactive material layer and a gel electrolyte layer formed thereon. 12.The method for preparation of a gel electrolyte battery according toclaim 1 wherein, said battery device preparation step (a), comprises:(a1) layering a strip-like positive electrode on each surface of apositive electrode collector, said positive electrode comprising (i) apositive active material layer comprising a lithium compound oxide and(ii) a gel electrolyte layer formed on said positive active materiallayer, and said positive electrode collector comprising a metal foil;(a2) layering a strip-like negative electrode on each surface of anegative electrode collector, said negative electrode comprising (iii) anegative active material layer comprising a material capable ofdoping/undoping lithium and (iv) a gel electrolyte layer formed on saidnegative active material layer, and said negative electrode collectorcomprising a metal foil; and (a3) layering together and coilinglongitudinally said strip-like positive electrode and the strip-likenegative electrode to form a battery device.